This disclosure relates to tower cranes, which are used for high lifts of loads, particularly in building erection. Tower cranes are characterized by a vertical tower, in contrast to the inclined booms of conventional cranes. The pivoted upper jib of a conventional crane is replaced by a much longer pivoted boom that is raised or lowered to place the load at the desired radial position relative to the tower axis.
Conventional tower cranes are counterweighted on the supporting carriage at the base of the tower. The overturning moment produced by the weight of the load multiplied by its distance from the tower axis is counterbalanced by a heavy counterweight spaced to the opposite side of the tower axis on the tower base or carriage. The overturning moment must always be smaller than the counterbalancing moment. This limitation restricts the maximum load permitted on the tower crane at a given radius. The maximum available separation between the counterweight and tower is limited by structural considerations when the counterweight is cantilevered from the supporting carriage at the base of the tower. In practice, this maximum spacing between the counterweight and tower axis is limited to about ten to fifteen feet. These spatial and structural limitations have in turn limited the amount of design load which can be suspended from a conventional tower crane.
Another common problem with respect to tower cranes is the tendency of the unloaded boom to fall backward over the tower due to the moment applied to it by the supporting rigging. It is common practice to use safety straps or cables between the tower and boom to limit the upward angle of the boom relative to the vertical tower axis. These straps or cables effectively prevent the boom from being pulled too high when unloaded but also limit the minimum radius at which loads can be lifted.
While the use of safety straps or cables between the tower and boom are not unduly restrictive in a conventinal tower crane, where the counterweight and attached rigging is not appreciably to the rear of the tower, much more serious problems are encountered when designing a tower crane having a counterweight spaced a substantially greater distance to the rear of the tower. Such spacing is made practical by supporting the counterweight on an independent self-powered crawler platform. This requires use of an elongated staymast that extends rearwardly or opposite to the boom. The potential moment exerted on the raised, unloaded boom by the combined weight of the staymast and rigging would require safety straps or cables beyond practical strength limitations, or would require such severe angular limitations on the operation of the boom as to make its utilization impractical in many applications.
As an example of a tower crane using a conventional counterweight assembly on a supporting carriage, and illustrating the use of safety cables between the boom and tower, reference is made to the disclosure of U.S. Pat. No. 3,794,184, which was issued to Robert J. Higgins on Feb. 26, 1974. The disclosure of this patent is incorporated herein by reference.
In my co-pending U.S. patent application, Ser. No. 877,816, filed Feb. 14, 1978, there is disclosed a tower crane having a rigid strut extending between a rearwardly spaced counterweight unit and the vertical tower. The counterweight unit is structurally connected to the base of the tower by a horizontal spreader or stinger which completes a triangular frame. The horizontal and diagonal legs of the triangular frame transmit rotational forces from the self propelled counterweight unit to both the base and upper end of the vertical tower. This earlier apparatus required a lifting crane for erection. While it was usable by itself where the boom could always be maintained at low angles relative to the tower, many practical applications of this structure required its use in conjunction with a guy derrick to serve as a boom stop beneath the staymast.
According to the present invention, a rigid counterweight strut extends between the staymast and the remote counterweight unit. It replaces the usual backstay lines and serves as a tension member when a load is being lifted at the outer end of the boom. More importantly, it eliminates the need for excessive safety straps or cables between the boom and tower and allows the unit to be designed with light safety straps or cables adequate only to prevent the boom itself from falling backwards over the tower. The counterweight strut also serves an important function during erection of the apparatus in providing the necessary lifting forces to make the structure substantially self-erecting.